Quantum computing is showing increasing promise. One of the main issues is that of entanglement. Being able to entangle photons has long been thought a necessity of quantum computing. In the nonlinear-optical process of spontaneous parametric down-conversion (SPDC), in which a laser beam illuminates a nonlinear-optical crystal, pairs of photons are generated in a state that can be entangled concurrently in polarization and in spatial modes.
Currently, interferometers which generate spatial entanglement in free space encounter significant disadvantages. For example, path length stability is a concern. Interferometric manipulations for the spatial modes are effectively a large web of interference paths whose phases are difficult to maintain stably in free space.
Accordingly, there is a need in the art for chip-scale interferometers which enable the generation and analysis of hyperentanglement which are phase stable.